The Atom Syndication Format defines an XML element named “link” that contains a “rel” and an “href” attribute (among others). Possible value for the rel attribute are either URIs (IRIs, to be exact) or simple text strings. In the latter case, they have to be one of the values registered at IANA; they should be considered equivalent to the URI created by appending the value to “http://www.iana.org/assignments/relation/” (which makes me wonder why IANA is unable to map this URI to the registry, but I digress).

I’m wondering about the “self” relation. According to the IANA registry, its definition is the one from Atom, which says

The value “self” signifies that the IRI in the value of the href attribute identifies a resource equivalent to the containing element.

The reason for my wondering is that I just re-read Subbu’s post on using URIs for identity. He argues there’s a problem because in using URIs for identity because you might retrieve the “same” object in different ways – e.g. you might retrieve a person’s information in two ways, just with the basic data:

GET /person/abc  
Host: www.example.org  

200 OK  
Content-Type: ...  

<person>  
  <link href="http://www.example.org/person/abc" rel="self"/>  
  <link href="http://www.example.org/person/abc?include=addressbook"  
    rel="http://www.example.org/rels/person-with-addressbook"/>  
  <first-name>Subbu</first-name>  
  <last-name>Allamaraju</last-name>  
  <email>[email protected]</email>  
  ...  
<person>

or including some additional information:

GET /myapp/person/abc?include=addressbook  
Host: www.example.org  

200 OK  
Content-Type: ...  

<person>  
  <link href="http://www.example.org/person/abc?include=addressBook" rel="self"/>  
  <first-name>Subbu</first-name>  
  <last-name>Allamaraju</last-name>  
  <addresses>  
    <address>  
      ...  
    </address>  
    ...  
  </addresses>  
<person>

The problem, as described by Subbu, is this:

Let me start with the “self” links. The person has a self link in each case, but they are all different. The client can not determine that the person with name Subbu Allamaraju found in the search results is the same as the one in the first or the second response. So, self links are useless to implement these scenarios.

But is this really a problem? The way I’ve used “self” links so far is to refer to a canonical resource, i.e. the one that represents the object or entity itself in the default way.

In other words, I’d turn Subbu’s second example into this:

GET /myapp/person/abc?include=addressbook  
Host: www.example.org  

200 OK  
Content-Type: ...  
<personWithAddressBook>
  <link href="http://www.example.org/person/abc?include=addressBook" rel="self"/>  
  <person>  
    <link href="http://www.example.org/person/abc" rel="self"/>  
    <first-name>Subbu</first-name>  
    <last-name>Allamaraju</last-name>  
  <person>
  <addresses>  
    <address>  
      ...  
    </address>  
    ...  
  </addresses>  
</personWithAddressBook>

Arguably, another option could be:

GET /myapp/person/abc?include=addressbook  
Host: www.example.org  

200 OK  
Content-Type: ...  

<person>  
  <link href="http://www.example.org/person/abc" rel="self"/>  
  <first-name>Subbu</first-name>  
  <last-name>Allamaraju</last-name>  
  <addresses>  
    <address>  
      ...  
    </address>  
    ...  
  </addresses>  
<person>

This has the downside that retrieving the resource identified by the outermost self link would yield something different than what you’re looking at. Whether this is acceptable or not depends on whether you consider these two resources to be “identical”. I’m undecided whether the official definition of the self link relation allows for this usage.

But in conclusion, I stand by my opinion that URIs can and should be used for identity – whatever “identity” might mean for you.